Germanium and phosphorus oxides, GeO.sub.2 and P.sub.2 O.sub.5, are nowadays the dopants most widely used to raise the refraction index of silica. Such dopants, however, have a number of disadvantages. Phosphorous oxide increases attenuation at high wavelengths due to molecular vibrations. In fact its presence gives rise to an absorption band beginning at about 1.3 .mu.m. Germanium oxide, in addition to its high cost due to its scarcity in nature, has high volatility at MCVD process working temperatures which limits its incorporation into the silica matrix.
Owing to the latter disadvantage, a decrease in the refractive index occurs at the center of the preform core, the so-called "dip", which in the case of multimode fibers limits their transmission band while in the case of single-mode fibers gives rise to problems if particular refractive indices are required, namely in the case of fibers with optimized dispersion between 1.3 and 1.5 .mu.m (dispersion shifted or dispersion flattened).
Alternative silica dopants, other than the above ones, could be Al.sub.2 O.sub.3, ZrO.sub.2, PbO, SnO.sub.2, GaO, Sb.sub.2 O.sub.3, etc. Alumina (Al.sub.2 O.sub.3) is less volatile at the working temperature of the MCVD process and can further raise the total refractive index of fiber core, even though it is implanted in limited concentrations (in fact at a concentration of 15% by weight in the matrix of SiO.sub.2) the refractive index passes from 1.4584 to 1.4630, with a 0.3% increase.
However, chemical compounds of metals, such as aluminum, are difficult to handle in the MCVD process, since no organic compounds exist which at room temperature have a high vapor pressure and hence can be easily maintained at the gaseous state up to the reaction zone where the solid is produced for fabricating the optical fiber preform.
An alternative solution could be that of starting from organometallic compounds such as described in the commonly owned Italian Patent Application 67338-A/85 corresponding to U.S. Pat. No. 4,738,873. These components have at room temperature high vapor pressure; however they are dangerous owing to their tendency to explode in the presence of oxygen.
Various processes of deposition of Al.sub.2 O.sub.3 --SiO.sub.2 --based matrices for fabricating optical fibers by the MCVD technique starting from AlCl.sub.3 have been described in the literature.
According to one of these processes, described by J. R. Simpson et al in the article entitled "Optical Fibers with an Al.sub.2 O.sub.3 --doped Silicate Core Composition" (Electronics Letters 19 Jul. 1983, 261-2), AlCl.sub.3 is sublimed directly at 130.degree. C.-180.degree. C. in a suitable vessel and is carried through a heated conduit into the reaction zone.
A second process described in British Patent Application 2,118,320 A, resides in producing AlCl.sub.3 or ZrCl.sub.4 in a reactor starting from Al or Zr and from Cl.sub.2 and diluents. The halogen is brought into contact at high temperature with metal wires to yield a gaseous halide compound.
Such processes, however, require the lines and rotating joints be heated to keep the halide at the vapor state from the generator to the reaction zone. The heated lines are cumbersome, difficult to maintain and present serious sealing problems in order to avoid pollution in the process reaction and deposition zones.